Process of esterifying cellulose fibers



Patented May 13, 1947 UNITED STATES PATENT OFFICE PROCESS OF ESTERIFYINGCELLULOSE FIBERS No Drawing. Application February 5, 1943, Serial No.474,894. In Switzerland January 14, 1942 7 Claims. 1

The esterification of regenerated cellulose with anhydrides of organicacids with retention of structure is known; in this connection, compare,among others, the U. S. Patents Nos. 1,922,287, 1,930,895 and 2,103,018and the French Patents Nos. 755,267, 758,185, 770,806 and 783,303 aswell as the Swiss Patent 157,681.

In the technical execution of this esterification on a large scaledifliculties have previously been encountered on account of the lack ofa suitable continuously working process. Naturally, a process of thisnature must be characterized by a reaction time which is as short aspossible and must work as rationally as possible in regard to thequantity of esterifying agent consumed. However, a short reaction timenecessitates working at high temperatures; but if esterifying baths areexposed for a long period to high temperatures in the presence ofcellulose and an esterifying catalyst, they will become unusable in acomparatively short space of time, due to discolouration, accumulationof impurities, and alteration of the composition. A continualreplacement of the esterifying bath is therefore necessary. From thiscause, a large volume of used baths accumulates which must beregenerated; this implies a heavy economic burden for the process.

If, in order "to avoid the difliculties mentioned, work is carried outusing esterifying agents in the gaseous state, other and no less seriousdifiiculties are met with. The esterifying reaction is an exothermicprocess and, when working in the gaseous phase-in contrast toesterifying in a liquid medium-local overheatings easily take place,causing irregularities -in the material. Furthermore, the boiling pointsof the organic acid anhydrides coming in question are very high, so thatthe work must be carried out under reduced pressure in order to avoidhaving to carry out the esterification at exaggeratedly hightemperatures. It is unnecessary to explain that extraordinarydifficulties would be met with if a continuous process had to be carriedout under reduced pressure.

It has now been found that it is possible to carry out theesterification in a short time, using small quantities of esterifyingagent, if regenerated cellulose (in continuous filament form or in formof staple fibres) which is loaded with a suitable catalyst beimpregnated or otherwise loaded with a liquid esterifying agent andexposed to the action of hot gases which are in a state of motion. Thehot gases are preferably circulated in a closed apparatus, so that theesterification is carried out in a gas circulation which is saturatedwith the esterifying agent. It has been ascertained--and this was not tobe foreseenthat, by this method of working, an extensive andsurprisingly regular esterification of the regenerated cellulose takesplace in a short time, and this without any of the disadvantages beingmet with which would otherwise occur when working with gaseousesterifying agents.

In order to be able to carry out the esterification within the requiredshort period of reaction it is preferable that a comparatively hightemperature, for example, somewhat in the neighbourhood of C. or higher,should be chosen. By a short time is to be understood a period which isshorter than 30 minutes, preferably shorter than ten minutes, since forlonger periods of time the size of the continuous apparatus would becometoo great.

The quantity of esterifying agent used doesv not need to be great; ingeneral, the quantity of acid anhydride which remains in the regeneratedcellulose after impregnation with the liquid anhydride and squeezing issufiicient. Loading with liquid esterifying agent can take place byimpregnation, irrigation or spraying, or in any other manner.

The incorporation of the catalyst in the regenerated cellulose takesplace before the esterification. As catalysts, those mentioned in thepatents listed above may be employed; especially suitable for anesterification at high temperature are alkali metal salts of weak acids,such as lithium, sodium and, particularly, potassium acetate. Theregenerated cellulose can be impregnated with an aqueous solution ofthese salts and the water removed by drying. Other catalysts and othermethods for their incorporation into the regenerated cellulose can alsobe chosen. When using a catalyst which is dissolved in water or in asolvent containing hydroxyl groups, the solvent must be removed beforethe esterification, either by drying or by displacement by a suitableliquid, so that no large quantities of anhydride will be used up byreaction with the solvent.

A particularly advantageous form of the process is that in which theexcess acid anhydride, i. e., the anhydride which is not'used in theesterifying reaction, is separated by cooling and thus recovered at asuitable point in the circulatory system. The continuous esterificationcan be carried out according to the present invention on all types ofstructures consisting of dissolved and reprecipitated cellulosematerials, no matter whether these be continuous structures offilaments, threads, woven materials, knitted materials, films ordiscontinuous structures, for example, staple fibre in the form offlocks. In the last case, the loose material must be conveyed throughthe continuous apparatus by means of a conveyor belt or other suitablearrangement. As diSSOlVed and reprecipitated cel lulose materials therecome into consideration first of all regenerated cellulose, such asviscose, cuprammonium and Chardonnet structures; but there can also beused as parent material dissolved and reprecipitated cellulosederivatives which contain reactive hy'droxyl groups, 'e. g.,incompletely esterified or etherified cellulose, 'for example dissolvedand reprecipitated partially acetylated or low methylated cellulose.

As esterifying agents, use may be made of-ac'id anhydrides of organicacids, such as acetic anhydride, propionic anhydride, butyric anhydride,or of mixed anhydrides, such as acetic-butyric anhydride, or of mixturesof anhydrides. They can be used dissolved in a suitable solvent,preferably however, undiluted.

As a gas which is caused to act in a hot state on the materialimpregnated with a catalyst and an organic acid anhydride, air ispreferred above all; however, there may also be used other gases,especially iridifierent gases, such as, for example, nitrogen.

The following examples illustrate the new process but are not to beregarded as limiting it in any way:

Example 1 Viscose solution is spun in known manner on a staple fibreapparatus to form a rope of parallel viscose filaments, and this latter,before drying, is passed through a solution of potassium acetate of 40per cent. strength. The rope, impieg'n'ated with potassium acetate anddried, is led through a vessel containing undiluted acetic animate; onleaving the vessel it passes between two adjustable squeezing rollers,the pressure of the rollers being adjusted to the desired degree ofesterification and to other conditions of the esterifying process. Therope, loaded with acetic anhydride, passes from the squeezing rollersinto the interior of a closed apparatus, in which it is led through aseries of driven pairs of rollers. During this passage it is exposed tothe-action of hot air which is circulated through the apparatus by meansof a fan. In this section of the apparatus the actual esterificationtakes place. The time taken by the esterification reaction and thus thespeed of passage of the filaments is dependent on the temperature ofreaction and can amount, for example, to 3 minutes at 110 C. or to 6minutes at 100 C. In 'a further section of the apparatus the filameritsmeet with a new air circulation, in the path of which are to be found acooler and an air heater. In this section of the apparatus the excessacetic anhydride, which is still being carried by the rope of filamentsis removed by the hot air and is separated in the cooler. cooled air isagain heated in the air heater and is again blown on to the filaments bymeans of the fan, preferably in the 'form of a countercurrent. Apartfrom the recovery of the excess acetic anhydride, a certain portion ofthe esterification can also take place in this section of the apparatus.

The esterified rope of filaments, which still contains potassium acetateand a'certain amount or acetic acid (formed during the esterifying Thereaction) combined as acid potassium acetate, is washed, after leavingthe esterifying apparatus, in a series of successive baths with water,the counter-current principle being adopted, after which it is finishedand dried. In this manneifl-a continuous rope of filaments, consistingno longer of regenerated cellulose but of acetyl cellulose, is obtained.The filaments can be cut to staple fibre if desired at any stage of theprocess, for example, directly before the drying or just as they leavethe esterification apparatu s.

In place of acetic anhydride, use may be made of butyric anhydride or ofthe mixed aceticbutyric anhydride in an analogous manner, allowancebeing made for the reduced reactivity of the anhydrides mentioned byincreasin the temperature of reaction, extending the reaction time oraddition of an accelerator, such as pyridine.

Example 2 Work is carried out as described in Example 1, the onlydifference being that a woven ribbon of rayon is esterified in place ofthe rope of filaments. The ribbon mustbe Woven lighter than usual, inorder that it will not become too dense on account of the increase involume of the individual fibres, caused by the esterification.

Example 3 Work is carried out as described in Example 1, with thedifference that, in place of the rope of filaments, staple fibreconsisting of regenerated cellulose in the already cut state, that is,in the form of flocks, is Subjected to the treatment. The staple fibre,impregnated with potassium acetate and dried, is carried in a looselayer on a conveyor belt; it is sprayed with acetic armydride, theexcess being removed by suetioii or by squeezing, and is treated in theesterification apparatus with hot air in a state of motion exactly asdescribed in Example 1. In place of several vertically arranged pairs ofrollers, horizontal conveyor bands, arranged one above the other andrunning in alternating opposite directiohs, are used to convey thestaple fibre. The flocks, on reaching the end of one conveyor band, fallon to the band below and are thereby turned over, which ensures aregular course of the es'terifylng reaction. After removal and recoveryof the excess acetic anhydride, the esterified fibres are scrooped anddried in a manner analogous to that described in Example 1.

What I claim is:

1. In the process for the continuous esterification of dissolved andrepr'ecipitated cellulose ma terial with retention er its structure toproduce at least monoacylat'e'd cellulose esters which are insoluble inorganic solvents by impregnating the material with a solution of acatalyst, removing the solvent, impregnatin with a liquid anhydrld'e ofan organic acid containing at the most 4 carbon atoms, and heating toeffect the 'acylation, the special method of working which comprises ofsaid liquid anhydride followin the impregna tion with the latter andprior to the heating step, and subjecting the separated and still unaclated impregnated material to the action of a hot circulating gas, whichis inert to the desired re action, at a temperature of at least aboutC., unti1the desired acylation is efiected.

2. in the process for thecontinuous esterification of dissolvedandreprecipitated cellulose material with retention of its structure toproduceat least monoacylated cellulose esters which are insoluble inorganic solvents by impregnating the material with a solution of acatalyst, removing the solvent, impregnating with a liquid anhydride ofan organic acid containing at the most 4 carbon atoms, and heating toeffect the acylation, the special method of working which comprisesseparating the impregnated material from excess of said liquid anhydridefollowing the impregnation with the latter and prior to the heatingstep, and subjecting the separated and still unacylated impregnatedmaterial to the action of a hot circulating gas, which is inert to thedesired reaction, at a temperature of about 100-110 C. for a period ofless than 30 minutes, until the desired acylation is effected.

3. In the process for the continuous esterification of regeneratedcellulose in continuous filament form with retention of its structure toproduce at least monoacylated cellulose esters which are insoluble inorganic solvents by impregnating the material with a solution of acatalyst, removing the solvent, impregnating with a liquid anhydride ofan organic acid containing at the most 4 carbon atoms, and heating toeffect the acylation, the special meth'od of working which comprisesseparating the impregnated material from excess of said liquid anhydridefollowing the impregnation with the latter and prior to the heatingstep, and subjecting the separated and still unacylated impregnatedmaterial to the action of a hot circulating gas, which is inert to thedesired reaction, at a temperature of about 1001l0 C. for a period ofless than 30 minutes, until the desired acylation is effected.

4. In the process for the continuous esterification of viscose in theform of staple fibers with retention of its structure to produce atleast monoacylated cellulose esters which are insoluble in organicsolvents by impregnating the material with a solution of a catalyst,removing the solvent, impregnating with a liquid anhydride of an organicacid containing at the most 4 carbon atoms, and heating to effect theacylation, the special method of working which comprises separating theimpregnated material from excess of said liquid anhydride following theimpregnation with the latter and prior to the heating step, andsubjecting the separated and still unacylated impregnated material tothe action of a hot circulating gas, which is inert to the desiredreaction, at a temperature of about IOU- C. for a period of less than 30minutes, until the desired acylation is efiected.

5. The process according to claim 1, wherein the catalyst is an alkalimetal salt of a weak acid.

6. The process according to claim 1, wherein the catalyst is an alkalimetal salt of a weak aliphatic acid and the anhydride is aceticanhydride.

7. The process according to claim 1, wherein the catalyst is potassiumacetate and the anhydride is acetic anhydride.

ANDREAS RUPERTI.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,930,895 Haller et al Oct. 17,1933 1,958,315 Rheiner May 8, 1934 2,103,018 Ruperti Dec, 21, 19372,172,475 Kaase et a1 Sept. 12, 1939 1,861,320 Rheiner May 31, 19322,270,658 Linnhoff Jan. 20, 1942 2,343,920 Maxwell Mar. 14, 1944 224,330Cross et al Sept. 28, 1907 FOREIGN PATENTS Number Country Date 5,016Great Britain 1907 419,918 Great Britain Nov. 21, 1934 540,790 GreatBritain Oct. 30, 1941 622,304 France May 28, 1927

